Safety system for flywheel system for mobile energy storage

ABSTRACT

A flywheel safety support system isolates the flywheel and its motor-generator from the driving environment of an electrically powered motor vehicle. A suitable liquid, placed between the outer and vacuum housings of the flywheel assembly, provides buoyancy and damping to the vacuum housing, cooling of the motor-generator, and serves as one of the barriers to rotor energy and angular momentum transfer in the event of an accident or failure. During normal operation, a shearable mechanical gimbal system keeps the vacuum housing centered in the outer housing, reacts the spin moments generated by the motor-generator, and provides a path for the electrical leads into the vacuum housing. In the event of bearing seizure or rotor failure, the mechanical gimbal will shear and allow the vacuum housing to gradually spin down against the fluid. A fiber barrier provides an additional layer of protection to passengers.

This is a Continuation of Ser. No. 08/637,649 (PCT/US94/11809), whichwas filed on Apr. 30, 1996, now U.S. Pat. No. 5,767,595, which, in turn,is a combined Continuation of Ser. No. 08/148,361, which was filed onNov. 8, 1993, now U.S. Pat. No. 5,559,381, and entitled "FLYWHEELSUPPORT SYSTEM FOR MOBILE ENERGY STORAGE", Ser. No. 08/242,647, whichwas filed on May 13, 1994, now U.S. Pat. No. 5,628,232, and entitled"FLYWHEEL ROTOR WITH CONICAL HUB AND METHODS OF MANUFACTURE THEREFOR",which is a Continuation-in-Part of application Ser. No. 08/181,038 filedJan. 14, 1994, now U.S. Pat. No. 5,566,588; also entitled "FLYWHEELROTOR WITH CONICAL HUB AND METHODS OF MANUFACTURE THEREFOR", and Ser.No. 08/199,897, which was filed on Feb. 22, 1994, now U.S. Pat. No.5,462,402, and entitled "FLYWHEEL ENERGY STORAGE SYSTEM WITH INTEGRALMOLECULAR PUMP".

FIELD OF THE INVENTION

The present invention relates generally to a flywheel energy storagedevice. More specifically, the present invention is related to aflywheel-motor-generator combination providing surge power, dynamicbraking, and energy storage for a hybrid electric motor vehicle. Thepresent invention is particularly advantageous when adapted for use in ahybrid electric motor vehicle.

One aspect of the present invention relates to the maintenance of avacuum within the space occupied by a high speed flywheel rotor. Morespecifically, the use of a molecular pump incorporated into the flywheelassembly of a flywheel energy storage system to pump gases from a rotorenvironment into a separate chamber is disclosed. The separate chamberadvantageously can contain molecular sieves for adsorbing gas moleculesgiven off by the rotor.

BACKGROUND OF THE INVENTION

The manufacture of electric vehicles powered by chemical batteries isbeing encouraged by air quality control agencies in an effort to reducethe air pollution created by the internal combustion engines in currentuse. Even though the electric power utilities which supply the energyused to charge the batteries are themselves polluters, the net result isfavorable with respect to air quality. However, the relatively poorcharacteristics of chemical batteries, in terms of weight, cycle life,and cost make it difficult for them to compete in the market place withinternal-combustion engines as the power system of choice.

A hybrid electric power train, consisting of a turbo-generator whichgenerates the average power consumed by the vehicle, a flywheel surgepower generator, an electric traction motor, and an electronic powercontrol system can achieve the low pollution levels needed for good airquality, but with performance characteristics which exceed those of theinternal combustion engine. Even though the turbine burns hydrocarbonfuels, its use of a catalytic combustor results in less air pollutionthan that created by the utilities which provide the electricity neededto charge the chemical batteries in vehicles so powered. The separationof the power sources into elements separately optimized to supply theaverage and the peak power, respectively, coupled with the ability touse dynamic braking, causes the efficiency over most driving schedulesto be enhanced and, thus, less fuel is consumed.

A description of a turbo-generator suitable for use in a hybrid electricvehicle is given in a paper by Robin Mackay for the SAE InternationalCongress and Exposition, March, 1994, entitled "Development of a 24 KWGas Turbine Generator Set for Hybrid Vehicles", which paper isincorporated herein by reference for all purposes. Many different typesof electric motors have been used for traction of electrically propelledvehicles for over a century. The present disclosure relates to thedesign of the flywheel energy storage system. The electric power controlsystem, the fourth major element of the electric power train, isdescribed in a co-pending U.S. patent application Ser. No. 08/246,240,which is entitled "ELECTRIC POWER TRAIN CONTROL" and which isincorporated herein for all purposes.

Modern high strength-to-weight ratio fibers make it possible toconstruct high energy density flywheels, which, when combined with ahigh power motor-generators, are an attractive alternative toelectrochemical batteries for use as energy buffers in hybrid electricvehicles. A properly designed flywheel system would provide higherenergy density, higher power density, higher efficiency, and longer lifethan a conventional electrochemical battery.

The vehicle environment, however, presents special challenges tosuccessful implementation of a flywheel to motor vehicle applications.Among these challenges are the need to deal with the gyroscopic torquesresulting from the vehicle's angular motions and the need to accommodatethe translational accelerations of the vehicle. Several safety issuesresulting from the high energy and momentum stored in the flywheel alsoneed to be taken into account, as does the difficulty of cooling themotor-generator operating in a vacuum chamber. In addition, energyconservation considerations and user convenience dictate the requirementthat the flywheel storage system possess a slow self-discharge rate.

Flywheel energy storage systems have been proposed for many years; manyof the storage systems have even been proposed for use in motorvehicles. U.S. Pat. No. 3,741,034, for example, discloses a flywheelcontained in an evacuated sphere which is surrounded by a liquid andhaving various safety features. However, the '034 patent does notaddress waste heat production and the requirement for cooling themotor-generator. In addition, the '034 patent does not address itself tothe dynamics of the driving environment, or the minimization of thepower drain when parked. U.S. Pat. Nos. 4,266,442, 4,285,251 and4,860,611, on the other hand, disclose different ways of constructinghigh speed rotors. However, the above referenced patents do notrecognize, let alone describe, design features needed for compatibilitywith the environment of a motor vehicle.

Moreover, in order to accommodate a rim speed of about 1000 meters persecond, a housing containing the flywheel should be maintained at a verylow pressure, e.g., a pressure below 0.01 Pascal, to limit windagelosses. While this pressure can be readily achieved before sealing thehousing, the fiber composite materials used in the construction of highenergy density flywheels have a residual gas evolution rate which makeit difficult to achieve this desired degree of pressure, i.e., nearvacuum conditions, in a sealed container. Thus, continuous pumping ofthe evolving gases from the container is often needed. Most often, anexternal pump is employed to maintain the desired pressure.

U.S. Pat. Nos. 4,023,920, 4,732,529 and 4,826,393 describe variousimplementations of molecular pumps, which are a class of high vacuumpump wherein the dimensions of the critical elements are comparable tothe mean free path of the gas molecules at the pressure of interest. Twotypes are generally known, a turbo-molecular pump, which is similar inconstruction to an axial flow compressor in a gas turbine employinginterleaved rotor and stator blades, and a molecular drag pump, whichuses helical grooves cut in the stator, which, in turn, is disposed inclose proximity to a high speed rotor so as to direct gas flow throughthe pump. It will be appreciated that hybrid molecular pumps, whichpumps contains separate sections of each of these types or molecularpumps, are also known. More specifically, U.S. Pat. No. 4,023,920discloses a turbo-molecular pump using magnetic bearings to support thepump rotor at high rotational speeds. U.S. Pat. Nos. 4,732,529 and4,826,393 disclose hybrid molecular pumps in which a turbo-molecularsection is used on the high vacuum input side and a spiral groove dragpump is used on the discharge side.

All of these pumps are designed as self-contained systems, each with itsown shaft, bearing system and power source, i.e., motor. While thissolution is satisfactory for stationary systems, it is more difficult toapply in mobile applications because the space and weight for itsimplementation is not readily available.

As discussed above, flywheel systems currently being designed for mobileenergy storage are generally intended to replace batteries inelectrically powered vehicles. In such applications, multiple units areneeded to store the required energy, so that each motor-generator needsupply only a small portion of the vehicle's power. In systems where allof the surge power must be supplied by a single flywheel, the relativelylarge size of the single motor-generator makes it difficult to providethe needed energy density without reducing safety factors, e.g., forradial stresses, to unacceptable low levels or raising manufacturingcosts to exorbitantly high levels.

The above-mentioned U.S. Pat. No. 3,741,034 discloses rotor designsusing high strength-to-weight ratio filament wound composites inrelatively thin concentric cylinders, which cylinders are separated byradial springs. While this arrangement limits the radial stresses totolerable values, it is expensive to manufacture. U.S. Pat. No.3,859,868 discloses techniques for varying the elasticity-density ratioof the rotor elements to minimize radial stresses. On the other hand,U.S. Pat. Nos. 4,341,001 and 4,821,599 describe the use of curvedmetallic hubs to connect the energy storage elements to the axle.Additionally, U.S. Pat. No. 5,124,605 discloses a flywheel systememploying counter-rotating flywheels, each of which includes a hub, arim and a plurality of tubular assemblies disposed parallel to the hubaxis for connecting the hub to the rim while allowing for differentialradial expansion between the hub and the rim.

None of the latter references deal with the integration of a large, highpower motor-generator into the flywheel energy storage system currentlybeing designed for vehicles.

The present invention was, thus, motivated by a desire to provide animproved flywheel-motor-generator energy storage system suitable formoving vehicles. More specifically, the present invention was motivatedby a desire to correct the perceived weaknesses and identified problemsassociated with conventional flywheel energy storage systems.

SUMMARY OF THE INVENTION

The principal purpose of the present invention is to provide a flywheelenergy storage system that is optimized for the motor vehicleenvironment. According to one aspect of the invention, the flywheelenergy storage system provides substantial surge power needed toaccommodate transient load requirements associated with the automobile.

An object to the present invention is to provide isolation for theflywheel from the vehicle's angular motions.

Another object of the present invention is to provide support for therotor during omni-directional accelerations, while maintaining smallradial gaps between the spinning and stationary elements.

Yet another object of the present invention is to provide an efficientand compact cooling system for a high-power motor-generator.

Another object of the present invention is to provide protection for thevehicle in which it is contained from accidental release of storedenergy and angular momentum.

Still another object of the present invention is to provide an energystorage device having a slow self-discharge rate.

A further object of the present invention is to provide a system locatedwithin a sealed chamber for maintaining pressure below a predeterminedthreshold.

Another object of the present invention is to provide a pressureregulating system for a flywheel energy storage system disposed within asealed housing wherein a shaft of the flywheel drives a pump for movinggas molecules from a first chamber to a second chamber within thehousing.

Yet another object of the present invention is to provide a pressureregulating system for a flywheel energy storage system disposed within asealed housing wherein bearings supporting a shaft of a flywheelsupports rotating elements of a pump moving gas molecules from a firstchamber to a second chamber within the housing.

Still another object of the present invention is to provide a pressureregulating system for a flywheel energy storage system disposed within asealed housing wherein a pump for moving gas molecules from a firstchamber to a second chamber within the housing is provided at a lowincremental cost.

An additional object of the present invention is to provide a pressureregulating system for a flywheel energy storage system disposed within asealed housing wherein the pressure is maintained by adsorbing gasmolecules moving from a first chamber to a second chamber within thehousing on a molecular sieve.

Still another object of the present invention is to provide a highenergy density rotor.

Another object according the present invention is to provide a highenergy density rotor which includes ample space within its volume for alarge, relatively high power motor-generator.

Still another object according the present invention is to provide ahigh energy density rotor which can be easily manufactured.

Yet another object according the present invention is to provide a highenergy density rotor which can be manufactured at a reasonable cost.

These and other objects, features and advantages of the presentinvention are accomplished by a flywheel energy storage system includinga fiber composite energy storing rotor, a high-powered, liquid-cooledmotor-generator supported by ball bearings in an evacuated sphere, whichsphere floats in a liquid contained in an outer spherical housing. Theenergy storage system includes a flywheel-motor-generator assemblyhaving a low center of mass location with respect to the evacuatedsphere so as to provide a vertical orientation of theflywheel-motor-generator along a rotor axis.

These and other objects, features and advantages according to thepresent invention are provided by an integral flywheel energy storagesystem combining a molecular pump into a flywheel energy storage systemfor vacuum control purposes. The integral flywheel energy storage systemincludes a sealed housing, a baffle including an orifice dividing thehousing into a low pressure first chamber and a relatively high pressuresecond chamber, a shaft suspended between first bearings located in thefirst chamber and second bearing in the second chamber, the shaft beingdisposed within the orifice, a flywheel disposed within the firstchamber spinning at high speed, and a molecular pump operativelyconnected for driving by the shaft for pumping gas molecules from thefirst chamber to the second chamber. It will be appreciated that otherbearing arrangements for operatively supporting the shaft can be usedwithout departing from the spirit and scope of the present invention.

According to one aspect of the invention, the molecular pump is designedinto the flywheel assembly so as to permit the high speed motor, shaft,and bearing needed by the molecular pump to be supplied by componentsalready present in the energy storage system. Preferably, the molecularpump transfers the gases evolving from the flywheel rotor and itsenvirons into a separate chamber within the housing of the energystorage system, i.e., contained within the overall vacuum housing. Thischamber advantageously may contain so-called molecular sieve materialsdesigned to adsorb the most prevalent of the gases given off by theflywheel rotor. It will be appreciated that other getter materials mayalso be used throughout the vacuum housing to adsorb trace elements notadsorbed by the molecular sieves.

These and other objects, features and advantages according to thepresent invention are provided by a molecular pump disposed with asealed housing of a flywheel energy storage system, wherein the shaftsupporting the flywheel powers the molecular pump to maintain gaspressure in the vicinity of the flywheel rotor at or below apredetermined pressure producing negligible drag on the spinningflywheel. It will be appreciated that the molecular pump transfers gasmolecules generated by the flywheel rotor material to a receivingchamber which advantageously contains so-called molecular sieves, whichadsorb these gas molecules, thereby maintaining the pressure of thereceiving chamber at a predetermined second pressure.

These and other objects, features and advantages according to thepresent invention are provided by a rotor including a generallycylindrical outer portion for storing most of the energy, and a hubportion attaching the outer portion to the shaft. In an exemplary case,the hub portion includes an engineered metallic disc member which can beattached to the outer cylindrical portion via an inner cylindricalmember having a relatively short axial extent.

According to another aspect of the invention, the arrangement of rotorcomponents provides the desired geometric properties in a readilymanufacturable configuration.

These and other features and advantages of the present invention willbecome more apparent from the following detailed description, taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments are described with reference to the drawings,in which like elements are denoted by like numbers, and in which:

FIG. 1 is a cutaway sketch of a hybrid electric vehicle showingrespective elements of its power train;

FIG. 2 is a high level block diagram illustrating the power controlsystem of the vehicle shown in FIG. 1;

FIG. 3 is an illustration showing the general arrangement of a flywheelassembly according to the present invention;

FIG. 4A is a cross-sectional view taken perpendicular to the axis of theflywheel illustrated in FIG. 3, FIG. 4B is a sectional view of the discmember, which is included in FIG. 4A, is useful in understanding theconstruction and operation of the disc member, while FIG. 4C illustratesradial stress and FIG. 4D illustrates tangential stress in the discmember profiled in FIG. 4B;

FIG. 5 is a detailed illustration of the upper bearing assembly and itslubrication system of the flywheel illustrated in FIG. 3;

FIG. 6 is a detailed illustration which is useful in understanding theconstruction and operation of lower bearing system and the associatedlubrication system for the flywheel illustrated in FIG. 3;

FIG. 7 illustrates the molecular drag pump used to maintain adequatevacuum in the chamber containing the flywheel rotor for the flywheelillustrated in FIG. 3;

FIG. 8 is a detailed illustration of an exemplary mechanical gimbalsupporting the flywheel assembly shown in FIG. 3;

FIG. 9 is an exemplary illustration showing an external protectivebarrier and the external radiator; and

FIG. 10A and FIG. 10B are illustrations which are useful in explainingthe construction and operation of a squeeze film damper employed by theflywheel shown in FIG. 3 in the bearing of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the power train elements of a hybrid electric vehicle usinga flywheel 1 as an energy buffer. In this configuration, the flywheel 1provides surge power for accelerating the vehicle and for hill climbing,complementing the relatively low, steady power provided by afuel-burning power source 3, e.g., a turbo-generator set. The flywheel 1is also used to absorb energy by storing it during dynamic braking anddownhill driving. An electric motor 4 converts the electric power fromeither the flywheel 1 or power source 3 to mechanical motive power.Preferably, all of these elements are regulated by the electroniccontroller 2.

FIG. 2 is high level a block diagram of a power control system showinghow the electronic controller 2 regulates the vehicle's power flow inresponse to the driver's inputs, which inputs are supplied by theaccelerator pedal 5 and the brake pedal 6. Controller 2 channels powerto the drive motor 4 from the turbo-generator 3 during cruise conditionsand augments this power with power from flywheel 1 for accelerating orhill climbing. Controller 2 advantageously charges the flywheel 1 withpower from the drive motor 4 which is acting as a generator duringbraking or downhill driving. Preferably, controller 2 maintains thespeed of flywheel 1 within a predetermined range by charging it frompower source 3 to avoid its lower limit or giving flywheel 1 a highershare of the driving load to thus avoid the flywheel's 1 upper limit.Controller 2 also channels power from the flywheel 1 to the power source3 for starting. In FIG. 2, power leads are designated by solid lines andsignal leads are designated by dashed lines.

FIG. 3 is a cross-sectional view of the entire flywheel assembly showingthe general arrangement of its parts. An outer housing 8 surrounds theassembly and provides mechanical and electrical connections to thevehicle. The space between housing 8 and a vacuum housing 10 is filledwith a liquid 9 in which the vacuum housing 10 floats. It will be notedthat bearings 14 and 15 are part of the mechanical gimbal system 80,which advantageously is provided between housings 8 and 10. The gimbalsystem 80 is discussed in greater detail below while referring to FIG.8.

The rotating assembly 100 includes a metal shaft 18 and is supported byan upper bearing assembly 12 and a lower bearing assembly 16. A squeezefilm damper 145 operates in conjunction with the lower bearing assembly16. The rotating assembly 100 is powered by a motor-generator 17including rotor 21a and a stator 21b.

The stator 21b is in good thermal contact with the re-entrant portion 25of the vacuum housing, i.e., a metal cylinder 20 perforated with axialholes 20a, which provide passageways for flow of the liquid 9.Advantageously, alternate holes 20a can be used for upward and downwardflow. All holes 20a are connected together in the top section ofcylinder 25 but are separated at the bottom into respective inlet andoutlet manifolds 25a, 25b. Flow separator 10a, which advantageously hasa small clearance with respect to outer housing 8, causes the liquidwhich is pumped by an external pump 54 through an external radiator 55to first flow bi-directionally past the stator 21a, removing its heat,and then through the annular space between the outer housing 8 and thevacuum housing 10. It will be appreciated from FIG. 9 that radiator 55can be a heat exchanger cooled by a dedicated fan 56. It will also beappreciated from FIG. 3 that flow separator is positioned so as topermit fluid flow through member 25 at all but the severest angles ofvehicle operation. Since periods during which the vehicle negotiateslarge angles are expected to be extremely short, minimal flowinterruptions will not produce unacceptable temperature increases inmotor-generator 17.

Preferably, the relatively cool liquid 9 pumped from the radiator 55enters the flywheel 1 via the inlet port 36 and exits the flywheel viaoutlet port 37 to return to the radiator 55 via pump 54.

The fiber composite cylinder 11 of assembly 100 is connected to theshaft 18 by means of a metallic hub 22 and an axially short fibercomposite cylinder 24. Preferably, the metallic hub 22 is formed ofaluminum, although any metal, metallic composite or compound having asubstantially similar, i.e., similarly high, ultimate strength tomodulus of elasticity ratio can be used. The assembly 100 stores energyin the form of rotational kinetic energy, most of it in cylinder 11. Atoroidal magnet 23 advantageously can be provided to produce a liftingforce equal to the weight of the rotating assembly 100.

A molecular drag pump 26 pumps residual gases evolving from material inthe low pressure compartment 28 into compartment 27, which containsmolecular sieves 27a to adsorb these gases. These compartments areseparated by a metal disc 29.

FIG. 4A is a sectional view taken perpendicular to the axis of rotationof the flywheel 1 shown in FIG. 3, showing an aluminum hub 22 used toconnect the shaft 18 to the cylinder 11 through the intermediatecylinder 24. The hub 22, which is shown in the cross-section in FIG. 4B,has an axial thickness which decreases with increasing radius in itsmain portion 22a. It will be noted that the main portion 22a accountsfor the majority of the hub 22. This shape advantageously provides anearly constant stress at each point along the radius. It will beappreciated that this constant stress profile permits maximal radialgrowth in this respective portion of hub 22.

At an outermost portion 22b of the radius, the axial thickness increasesabruptly to thereby form a radially thin outer cylindrical section 22c.It should be noted that this cylindrical section 22c includesterminating pads 22d and 22e, which advantageously can be bonded to theintermediate composite cylinder 24 shown in FIGS. 3, 4A and 4B. It willalso be noted the cylindrical portion 22c flexes in response tovariations in applied centrifugal force. It will be understood that thecombination of the stretch of the main portion 22a with the flexibilityof the cylindrical portion 22c permits pads 22d, 22e to follow theradial growth of the cylinder 24 without overstressing any point of thehub 22.

Preferably, rotating assembly 100, which in an exemplary case is 12inches in diameter, stores approximately 2 kilowatt-hours, i.e.,7,200,000 joules, of energy at a maximum rotational speed of about 6500radians per second. It will be appreciated that this corresponds to asurface speed of about 1000 meters per second. It will be noted thatthis high speed dictates that the rotating assembly be enclosed in anevacuated container. Moreover, the high centrifugal accelerationsrequire that the rotating assembly 100 be constructed primarily of highstrength fiber composites, e.g., a filament wound in the circumferentialdirection.

Preferably, rotating assembly 100, which is shown in detail in FIG. 3,includes two major elements, an outer, primarily cylindrical portion 11,which in an exemplary case can be up to 12 inches long, and the metallichub 22. The inner composite cylinder 24 connects hub 22 with outercomposite cylinder 11. The outer composite cylinder 11, which is shownin FIG. 3, consists of two regions, an outermost region 11a, whichpreferably is a filament wound composite using the highest strengthgraphite fiber available to sustain the centrifugal acceleration of onemillion G's, and an innermost region 11b, which is a filament woundfiber composite, whose combination of density and modulus of elasticitycreate a moderate compressive load on the outermost member 11a. Thisadvantageously minimizes the radial tension in the outermost member 11a.The radial and tangential stresses achieved with this material are shownin FIGS. 4C and 4D, respectively, as discussed in greater detail below.

The highest strength graphite fiber, which is used in fabrication ofoutermost region 11a, advantageously has a minimum tensile strength ofabout 924,000 lb/in² (924 kpsi) while the wound fiber used in thefabrication of composite cylinder 24 has a tensile strength of about 450kpsi. The cylinder 24 advantageously can be manufactured using amaterial sold under the brand name "Spectra". It should be noted thatthe moderate strength graphite fiber used in innermost cylinder region11b has a minimum tensile strength of about 714 kpsi. High strengthaluminum with a minimum tensile strength of about 75 kpsi advantageouslycan be used in the construction of hub 22, as discussed in greaterdetail above.

The rotating assembly 100 advantageously can be fabricated as twoseparate pieces, the hub 22 and outer cylindrical portion including bothcylinders 24 and 11. These two pieces advantageously are then mated withan interference fit. It will be appreciated that the interference fitresults in compression of the terminating pads 22d, 22e in the directionof shaft 18.

The fiber properties in cylinders 24 and 11 important for thisapplication are tensile strength and modulus of elasticity. The radialstress in these cylinders, which extend from the inner radius ofcylinder 24 of 3.7 inches to the outer radius of cylinder 11 of 6inches, is shown in FIG. 4C to be less than 4000 pounds per square inchat the highest rotational speed, well within the capability of the epoxymatrix material. The matrix material alone bears this stress, since thefiber, being circumferentially wound, makes no contribution to theradial strength. The gradation of the modulus of elasticity of thefibers from 24 million psi in cylinder 24 to 33 million psi for theinner portion of cylinder 11b to 43 million psi for the outer portion ofcylinder 11a accounts for the shape of the radial stress curve and itsdesirably low maximum value.

The hoop stresses in the cylinders are shown in FIG. 4D. They are seento be a maximum of 100,000 psi in cylinder 24 and 200,000 psi incylinder 11. These stresses are borne by the fibers, and are well belowthe ultimate capabilities of the materials employed. The fiber used incylinder 24 has an ultimate tensile strength of 435,000 psi, which isreduced by the fill factor of two thirds in the composite to 290,000psi. The fiber in the inner portion of cylinder 11 has a reducedultimate strength of 476,000 psi, and the fiber in the outer portion hasa reduced ultimate strength of 616,000 psi. The factor of three instrength indicated allows for both degradation due to fatigue and asubstantial margin of safety.

The cylinder 11 is assembled onto cylinder 24 with an interference fit,as is the cylinder 24 onto the hub 22. This causes the hub to be incompression when the rotor is at rest, which reduces its radial growthand tension when the rotor is spinning. This technique allows the metalhub to match the radial growth of the composite cylinders without beingoverstressed.

FIG. 5 gives details of the upper bearing assembly 12. Preferably, anangular contact bearing 30, using ceramic balls 30a to provide longbearing life, supports the spinning shaft 18 disposed in vacuum housing10. Bearing 12 advantageously can be lubricated by means of acirculating oil system in which oil pumping action is provided by acombination of centrifugal and gravitational forces. When oil in aspinning reservoir 36, whose free surface forms a vertical cylinder whenthe shaft 18 is spinning, exceeds its desired level, a scoop 32connected to a stationary shaft 37 scoops the excess oil into stationaryreservoir 39. Preferably, the oil then flows by gravity from reservoir39 to central chamber 40. The oil thus collected is discharged tospinning chamber 35. Advantageously, the flow rate is regulated by theoil flow metering plug 34 through which the oil passes between centralchamber 40 and spinning chamber 35. Centrifugal force in spinningchamber 35 throws the thus-introduced oil radially outward. Thisadvantageously permits the flow of oil to pass through oil flow holes 33so as to enter the bearing 30. The centrifugal force in the rotatingportions of the bearing 30 slings oil into the spinning reservoir 36,thus permitting the cycle to begin anew.

It will be appreciated that the small gap 31 between the stationary androtating conical surfaces of bearing 12 shown in FIG. 5 acts as aneffective seal or trap which prevents oil droplets from escaping fromthe vicinity of bearing 12 into flywheel chamber 27. Any oil dropletswhich might enter gap 31 advantageously can be accelerated outwardly bythe spinning wall of conical member 41 and, thus, caused to reenter thespinning reservoir 36.

It should be noted that before shaft 18 begins to rotate, the oilresides in spinning chamber 35. Once shaft rotation begins, theabove-described oil circulation cycle begins.

FIG. 6 is an illustration which finds use in explaining the operation ofthe lower bearing assembly 16. Preferably, bearing 140 is of the angularcontact type which advantageously uses ceramic balls 140a to accommodatelong life, just as in the upper bearing 12. Bearing 140 can belubricated by a circulating oil system.

Preferably, the circulating oil system 130 includes a rotating disc 141which slings lubricating oil from the rotating part of bearing 140outward into a reservoir 142. It should be noted that the oil level inreservoir 142 is indicated by the dashed line. Lubricating oil flowsthrough hole 143 into a squeeze film damper 145, whose narrow annulusformed by concentric metal cylinders 145a, 145b contains a radial spring145c as well as lubricating oil. Details of the squeeze film damper 145are shown in FIG. 10, wherein FIG. 10A is an axial view of a small arcof squeeze film damper 145 illustrating the annular space betweenconcentric cylinders 145a and 145b occupied by radial spring 145c.

Preferably, radial spring 145c is a chemically etched part whose etchpattern is as illustrated in FIG. 10B. It will be appreciated that whenthe radial spring 145a is wrapped around cylinder 145a, the halfrectangles of the pattern will stick out substantially, forming hundredsof elementary springs whose ends contact the inner surface of cylinder145b. The space between the cylinders 145a, 145b not occupied by theradial spring 145c is filled with lubricating oil. Advantageously, thespring 145c gives a restoring force to counteract the radialdisplacement of the outer cylinder 145a, which is connected to thevacuum sphere 10, with respect to the inner cylinder 145b, which isrotably coupled to the spinning shaft 18 via bearing 140.

The presence of viscous oil in this annulus produces a radial forceproportional to the rate of this displacement. The squeeze film damper145 acts as a means for limiting the amplitude of vibrations at shaftcritical frequencies caused by residual unbalance of the rotatingassembly 100.

Referring to FIG. 6, lubricating oil enters reservoir 144 through hole149 at the bottom of squeeze film damper 145. It should be noted thatthe oil level in reservoir 144 is indicated by the dashed line.Lubricating oil enters the vertical hole 146 in spinning cone 150 andflows out through radial holes 147 to thereby impinge on the rotatingpart of bearing 140, and thereby begin its circulatory cycle anew.

Advantageously, a double Belleville washer 148 can be used to preloadboth bearing 12 and bearing 16. It will be noted washer 148 produces anaxial force on the curved races of bearings 12, 16, which advantageouslysqueezes the balls in each respective bearing radially. The stress thusproduced creates the desired area of contact between the balls and theassociated races, which, in turn, produces the desired radial stiffnessof the bearing assembly. It will be appreciated that since most of theservice life of the bearings is spent with the preload as the only load,the preload is kept as small as consistent with the radial stiffnessrequirement, thus maximizing bearing life.

FIG. 7 shows the construction of the molecular drag pump 26 whichadvantageously maintains the pressure in vacuum housing 10 at apredetermined pressure. It will be noted that gases slowly evolve fromthe flywheel materials. Preferably, molecular drag pump 26 pumps theoffending gas molecules from the chamber 28 in which the shaft 18 spinsinto chamber 27, which contains molecular sieves 27a. It will further benoted that molecular sieves 27a preferentially adsorb the pumped gasmolecules. This pumping action advantageously maintains the gas pressurein chamber 28 low enough to achieve low aerodynamic drag and, thus,minimize heat generation due to the spinning fiber composite cylinder 11of assembly 100, whose surface speed can easily exceed 1000 meters persecond. Drag pump 26 consists of a spiral groove on the inside of thestationary cylinder 38 in close proximity to the spinning shaft 18.Since the bearing assemblies 12, 16 and motor 17 used for powering dragpump 26 are those required for the flywheel 1, the additional cost ofadding this important function is negligible.

More specifically, a separate gas storage chamber 27, located proximateto one of the bearings 12, 16 is formed by a baffle plate 29. It will beappreciated from FIG. 7 that baffle plate 29 includes an orifice 29a forpositioning of the shaft 18. Preferably, the bearing 12 is disposedwithin molecular pump 26, which advantageously may be a molecular dragpump 26. Preferably, gas storage chamber 27 contains so-called molecularsieves 27a, which will be discussed in greater detail below.

The purpose of the present invention is to maintain a high vacuum in thespace in which the flywheel rotor spins so that a negligible drag on theflywheel rotating assembly 100 will be produced. It will be appreciatedthat at a preferred rim speeds of about 1000 meters per second, thepressure in housing 10 should be less than to 0.01 Pascal. It will alsobe noted that the fiber composite materials used in the construction ofhigh energy density flywheels, i.e., flywheel assembly 100, have apropensity for residual gas evolution at a rate which make it difficultto achieve this desired degree of vacuum in a sealed container.Therefore, continuous pumping of the evolved gases from the container inconventional systems is often performed using an external

In contrast to these conventional systems, a molecular pump, which isdesigned into the flywheel 1, and which employs the high speed motor,shaft, and bearing system already present in the flywheel energy storagesystem, transfers the gases evolving from the flywheel assembly 100 andits environs into a separate chamber 27, which chamber is fullycontained within the overall vacuum housing 10. Advantageously, chamber27 contains molecular sieves 27a designed to adsorb the most prevalentof the gases generated by, e.g., cylinder 11. Preferably, getters aredisposed throughout the vacuum housing 10 to adsorb trace quantities ofgases which are not readily adsorbed by molecular sieves 27a.

The flywheel assembly 100, in an exemplary case, is 12 inches indiameter and has a maximum rotational speed of 6500 radians per second.This rotational speed corresponds to a surface speed of 1000 meters persecond, which high speed requires that the surrounding gas pressure bemaintained at a pressure less than 0.01 Pascal in order to permit asufficiently long self discharge time.

It will be appreciated that even though the flywheel assembly 100 willbe exposed to a high temperature bakeout while vacuum housing 10 isbeing evacuated prior to being sealed, the high mass of the volatilematerials of the composites, particularly the epoxy, employed in theconstruction of flywheel assembly 100 can be expected to produce aresidual gas evolution rate which could exceed the allowable pressurefor the vacuum housing 10 in a relatively short time. The molecular dragpump 26 advantageously can be used to pump these gases into gas storagechamber 27 where the gases can be adsorbed by the molecular sieves 27a.It will be appreciated that the pressure in housing 10 can, thus, bemaintained in the vicinity of the flywheel cylinder 11, even though thepressure in the storage chamber 27 may rise as high as one Pascal.

It will also be appreciated that, e.g., molecular drag pump 26 would betoo expensive an item to be used for maintaining the pressure of housing10 below its maximum allowable pressure if molecular drag pump 26 wereto be provided as a self contained item, principally because of the costof the high speed bearings and motor required by stand alone molecularpumps of any configuration. By integrating molecular drag pump 26 intothe design of flywheel assembly 100, the shaft, bearings, and motor ofthe flywheel assembly 100 advantageously can be used by molecular dragpump 100. It will be noted that the incremental cost of incorporatingthe molecular pump into the flywheel energy storage system is very low.

Molecular sieves are adsorbents whose pores are tailored in size to thedimensions of the molecules to be adsorbed. They are available under thetrade name MOLSIV from the Union Carbide Corporation. Their ability toadsorb is strongly influenced by pressure, e.g., the adsorption abilityis low at the pressure normally applied to flywheel assembly 100. Itshould also be noted that at the normal operating pressure of gasstorage chamber 27, i.e., a pressure P₂ which is approximately onethousand times higher than a pressure P₁ felt throughout housing 10, themolecular sieves 27a are capable of adsorbing all of the gases evolvedfrom flywheel assembly 100. In other words, at the upstream pressure P₁of the molecular drag pump 26, the adsorption rate of the target gasmolecules produced by the flywheel assembly 100 is low. The adsorptionrate increases as the pressure P₂ in chamber 27 is increased.Preferably, molecular sieve material is selected so that a minimumadsorption rate, e.g., the minimum adsorption rate necessary to matchthe gas molecule evolution rate of flywheel assembly 100, is achieved ata pressure lower than the shut off head of the molecular drag pump 26.

Preferably, a helical groove 26a cut into the stator of drag pump 26provides the flow path for the evolved gases from the high vacuumchamber, at pressure P₁, e.g., 0.01 Pascal, to the chamber 27 containingthe molecular sieves 27a in which the pressure P₂ may be as high as 10.0Pascal.

It will be appreciated that an alternate embodiment of the presentinvention wherein a turbo-molecular pump 26' is substituted formolecular drag pump 26. The pump 26' consists of a multiplicity ofturbine blades connected to the shaft 18 of the pump 26', interleavedwith stator blades supported by plate 29. It will be appreciated thatpump 26' serves the same function as pump 26 in pumping gases evolvingfrom the flywheel rotor 100 into gas storage chamber 27 containing themolecular sieves 27a. Turbo-molecular pump 26' may be usedadvantageously with some flywheel configurations in which more space isavailable along the shaft than in the configuration shown in FIG. 3.

FIG. 8 illustrates the mechanical gimbal assembly 80, consisting of asteel band 50 in the annular space between the outer housing 8 andvacuum housing 10. Band 50 is attached to the vacuum housing 10 by meansof journal bearings 14 and 15, which are diametrically opposed to oneanother. A second set of journal bearings, 51 (shown) and 52 (not shown)also diametrically opposed to one another and are rotated by 90°(rotational degrees) from the first set of journal bearings 14, 15connected to the band 50 on the outer surface of vacuum housing 10. Thisarrangement isolates the vacuum housing 10 which contains the flywheelassembly 100 from pitch and roll angular motions of the vehicle. Themotor-generator torques are reacted by the gimbal 80, which alsotransmits the residual acceleration loads which result from the smalldeparture from neutral buoyancy of the vacuum sphere in the flotationliquid 9. The journal bearing shafts are sized to shear under the hightorque overloads which would occur in the event of a flywheel failurecorresponding to bearing seizure. This is a safety feature to preventthe flywheel from jerking the vehicle.

In addition to these functions, the gimbal assembly also providesmechanical support for the power leads which must be routed from theouter housing into the vacuum housing to connect to the motor-generator.

The operation of the flywheel-motor-generator assembly will now bedescribed in detail.

An object of the support system is to permit the flywheel 1 to safelyperform its function as an energy buffer during all driving conditions,while consuming negligible power when the vehicle is parked, even on asteep hill. Since the surface speed of the rotor 100 may exceed 1000meters per second at peak charge, the assembly 100 must be maintained ina vacuum. The small, oil lubricated ceramic ball bearings 30, 140 canprovide the desired service life provided the mechanical loads are keptas low as possible. The overall design of this flywheel system is aimedat minimizing these loads.

It will be appreciated that placing the vacuum housing 10 in a gimbalsystem 80 makes the flywheel 1 nearly impervious to vehicle rotations.If the flywheel 1 were not gimbaled, a vehicle rotation would cause agyroscopic torque of magnitude (HdP/dt), where H is the angular momentumof the flywheel 1 and dP/dt is the pitch or roll angular velocity of thevehicle. The reaction at each bearing of the unit depicted in FIG. 3,which preferably is capable of storing 2 KWH of energy at full charge,would be 6000 newtons per radian per second of vehicle pitch or roll. Itwill be appreciated that this represents a load that would shorten thelife of the bearings on all but the smoothest of roads. The use of thegimbal system 80 described above reduces the moments exerted on thebearings 30, 140 to those produced by hydrodynamic forces on the vacuumhousing 10 and the spring forces produced by the power leads. Becausethe liquid 9 provides nearly neutral buoyancy to the inner housing, themechanical gimbal need not support the bulk of the acceleration loads,i.e., these loads mainly are borne by liquid 9. The mechanical gimbalneed only react to the spin-up and spin-down torques developed by themotor-generator 17, which are 12.5 newton-meters when the flywheel 1 isdelivering or accepting 80 kilowatts of power at its quiescent operatingspeed of 6400 radians per second. Thus, gimbal 80 preferably can have asmall enough drag area to make the hydrodynamic torques it developsduring vehicle pitching and rolling negligibly small.

During steady driving the orientation of the rotor axis is vertical, aconsequence of the center of mass of the vacuum housing 10 and itscontents being below the center of buoyancy, which arrangementadvantageously produces a righting moment on vacuum housing 10. In thisorientation, the weight of the assembly 100 is borne by the toroidalmagnet 23 and the forces on the bearings are those produced by thepreload spring 148. This advantageously can be made as small as theradial stiffness requirement permits.

When the vehicle is accelerating or braking, the spin axis is no longervertical, aligning itself, after a transient, to the equivalentgravitational field which is the vector sum of the earth's gravitationalacceleration and the vehicle's acceleration. Thus, the bearing loadduring steady accelerations is primarily axial. During transients, whichcause a damped precessional motion of the axis, the bearings react tothe small torques associated with this motion by exerting radial forces.

When the vehicle is parked, even on a hill, the spin axis is very closeto vertical, just as in steady driving. The spring forces exerted by thepower leads routed along the gimbal system 80 produce a torque tendingto align the axis perpendicular to the hill, but these forcesadvantageously are small enough to keep the resulting offset fromvertical negligibly small. With a vertical orientation of the rotor axiswhen the vehicle is stationary, the rotor weight is exactly offset bythe magnet 23, thus minimizing the load on the bearings 12, 16, therebymaximizing bearing life.

Another object of the present invention is to provide adequate coolingof the motor-generator 17 under all driving conditions, the mostdemanding of which is a repetitive stop and go driving schedule. Duringthis cyclic use, the motor-generator 17 is alternately delivering poweras a generator when accelerating the vehicle or accepting power as amotor during dynamic braking. Even though it is advantageously veryefficient in both operating modes, the high powers involved, e.g., manytens of kilowatts, create iron and copper losses which would lead todestructive temperatures in the motor-generator 17 if cooling were notprovided.

Advantageously, one preferred embodiment according to the presentinvention provides effective cooling of the motor-generator stator 21aby circulating flotation liquid 9 through axial holes 20a in the metalcylinder 25, as previously described. Since the bearings 12, 16 providevery little thermal conduction from the rotating shaft 18, the rotor 21bof the motor-generator is cooled primarily by radiation. The shafttemperature needed for this thermal radiation can be maintained withinacceptable limits by using a motor-generator design which minimizesrotor losses, such as a synchronous reluctance machine. The relativelycool spherical boundary, i.e., the vacuum housing 10, into which therotating assembly 100 radiates helps keep the rotor temperature withinacceptable limits.

Another object of the present invention is to protect the vehicle andits passengers from (a) an accidental sudden release of the storedenergy or (b) transfer of angular momentum, events which could be causedeither by vehicle collision or by mechanical failure of the flywheel 1.Although the energy of a full charge is only equivalent to thatresulting from the burning of six ounces of gasoline, its potentiallydangerous form of release, i.e., sudden release, must be considered.Preferably, four barriers are provided between the rotating assembly 100and the outside: the vacuum housing 10, the liquid 9, the outerenclosure 8, and an outer wrapping of fiber composite material 52 whichsurrounds and supports the housing 8 using foam pads 53 in theintervening space. See FIG. 9.

The heat released by a full charge will produce an increase in thetemperature in the fluid of approximately a few hundred degrees, causingno significant hazard. The sudden transfer of the rotor's angularmomentum to the vehicle could jerk the vehicle dangerously, if such werepermitted to happen. This is precluded in the preferred embodiment ofthe present invention by allowing the vacuum housing 10 to spin downgradually in the liquid 9 when pins in the mechanical gimbal shear inthe event of bearing seizure or of rotor disintegration. This detail isshown in FIG. 8.

The foregoing description of a preferred embodiment of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and many modifications and variations are possible inlight of the above teaching. The embodiment was chosen and described inorder to best explain the principles of the invention and its practicalapplication to electric vehicles, thereby enabling others skilled in theart to best utilize the invention in various embodiments and withvarious modifications as are suited to the particular vehicle usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto.

What is claimed is:
 1. A safety system for a high energy densityflywheel assembly, comprising:a vacuum housing; an energy absorbingsuspension fluid; an outer housing; and a reinforcing wrapping, arrangedin the recited order and arranged to form a multiple barrier and energydissipation means for protecting a vehicle and associated passengersfrom accidental sudden release of energy from the flywheel assembly, andfurther comprising a mechanical gimbal including shearable gimbal pinsprotecting the passengers and the vehicle from sudden transfer ofangular momentum resulting from bearing seizure of at least one of aplurality of bearings or contact between said vacuum housing and saidflywheel assembly, wherein gimbal pins in said gimbal shear so as topermit said vacuum housing to gradually spin down against the fluiddisposed between said vacuum housing and said outer housing.
 2. Animproved integrated system for a flywheel-motor-generator assemblyproviding mobile energy storage and surge power generation, said systemcomprising:an outer housing a vacuum housing within said outer housingwherein said vacuum housing and said outer housing define a cavity; apump for circulating a liquid in a predetermined portion of said cavityso as to transfer waste heat to said radiator; and a radiator; saidimprovement comprising:a reinforcing wrapping; wherein said liquid is anenergy absorbing liquid, wherein said vacuum housing, said energyabsorbing liquid, said outer housing and said reinforcing wrapping arearranged in the specifically recited order, which arrangement forms amultiple barrier and energy dissipation mechanism protecting a vehicleand associated passengers from accidental sudden release of energy fromthe flywheel assembly, and wherein said radiator is disposed outside ofsaid reinforcing wrapping.
 3. The improved integrated system for aflywheel-motor-generator assembly as recited in claim 2, furthercomprising a mechanical gimbal including shearable gimbal pinsprotecting the passengers and the vehicle from sudden transfer ofangular momentum resulting from bearing seizure of at least one of aplurality of bearings or contact between said vacuum housing and saidflywheel assembly, wherein gimbal pins in said gimbal shear so as topermit said vacuum housing to gradually spin down against said energyabsorbing liquid disposed between said vacuum housing and said outerhousing.
 4. The improved integrated system for aflywheel-motor-generator assembly as recited in claim 2, wherein astator of said flywheel-motor-generator is disposed proximate to saidpredetermined portion of said cavity.
 5. The improved integrated systemfor a flywheel-motor-generator assembly as recited in claim 2, furthercomprising a plurality of shearable support members operatively disposedbetween said outer housing and said vacuum housing.
 6. The improvedintegrated system for a flywheel-motor-generator assembly as recited inclaim 2, wherein the flywheel-motor-generator assembly has a respectivecenter of mass, wherein said vacuum housing has an associated center ofbuoyancy and wherein said center of buoyancy is disposed with respect tosaid center of mass so as to produce a righting moment allied to theflywheel-motor-generator assembly.
 7. The flywheel assembly system asrecited in claim 6, wherein said righting moment is applied to a rotoraxis of the flywheel-motor-generator assembly so as to permit alignmentof the rotor axis along a vertical during steady driving or when thevehicle is parked, irrespective of the orientation of the vehicle. 8.The improved integrated system for a flywheel-motor-generator assemblyas recited in claim 2, wherein said liquid outside of said portion ispumped so as to maintain said vacuum housing wetted by said liquidoutside of said portion at close to ambient temperature.
 9. An isolationsystem for a flywheel-motor-generator assembly providing mobile energystorage having a support system including a liquid suspension systemcomprising an outer housing, a vacuum housing disposed within said outerhousing, wherein said vacuum housing and said outer housing define acavity capable of being filled with liquid, a radiator fluidly coupledto said cavity, and a pump operatively connected to said radiator andsaid cavity, wherein said pump, said radiator and said cavity define acooling loop whereby said liquid is circulated between said radiator andsaid cavity so as to transfer waste heat to said radiator, saidisolation system comprising:a reinforcing wrapping; and a mechanicalgimbal including shearable gimbal pins for protecting the passengers andthe vehicle from sudden transfer of angular momentum resulting fromeither bearing failure or contact between said vacuum housing and saidflywheel assembly, wherein said liquid comprises an energy absorbingsuspension liquid, wherein said vacuum housing, said cavity in whichsaid energy absorbing liquid and said mechanical gimbal are disposed,said outer housing and said reinforcing wrapping are arranged in thespecifically recited order to form a multiple barrier and energydissipation means for protecting a vehicle and associated passengersfrom accidental sudden release of energy or said angular momentum fromthe flywheel assembly, wherein said gimbal pins in said gimbal shear soas to permit said vacuum housing to gradually spin down against saidenergy absorbing suspension liquid disposed between said vacuum housingand said outer housing, and wherein said radiator is disposed outside ofsaid reinforcing wrapping.
 10. The isolation system as recited in claim9, wherein said vacuum housing includes a member thermally coupled to astator of said flywheel-motor-generator, said stator being disposedproximate to said member, and wherein said energy absorbing suspensionliquid flows through said member thereby providing cooling of saidflywheel-motor-generator.
 11. The isolation system as recited in claim9, wherein the flywheel-motor-generator assembly has a respective centerof mass, wherein said vacuum housing has an associated center ofbuoyancy and wherein said center of buoyancy is disposed with respect tosaid center of mass so as to produce a righting moment applied to theflywheel-motor-generator assembly.
 12. The isolation system as recitedin claim 11, wherein said righting moment is applied to a rotor axis ofthe flywheel-motor-generator assembly so as to permit alignment of therotor axis along a vertical when an associated vehicle is parkedirrespective of orientation of the vehicle.
 13. The isolation system asrecited in claim 9, wherein said liquid outside of said portion isforcibly circulated so as to maintain said vacuum housing wetted by saidenergy absorbing suspension liquid outside of said member at ambientliquid temperature.